This invention relates to a method for directly depositing a polarized material with a net polarization onto a substrate. This invention also provides a freeform fabrication method for making a multi-layered device that contains a polarized material which is essentially stable up to its crystal melting point (if the polarized material is crystalline) or its softening point (if the material is non-crystalline). The polarized material is substantially free of mechanically-induced orientation and has mechanical and electromechanical properties isotropic in a plane perpendicular to the poling field direction. The method is particularly useful for making a micro-electro-mechanical system (MEMS) featuring a polymeric material element with piezoelectric and pyroelectric properties.
Recent advancements in the microelectronics industry have allowed integrated circuit (IC) chip manufacturer to achieve a very high packing density within a single IC chip. However, the electronics packaging industry has not seen the same degree of size reduction as in the IC industry. One reason for this difference lies in the need to utilize discrete active and passive devices on circuit boards as well as electrical interconnections to achieve fully functioning IC devices. Due to the requirement of placing each of the discrete devices onto the circuit board, various physical constrains dictate the size that the circuit board must maintain.
In order to miniaturize and integrate traditional microelectronic elements and functionally responsive devices (e.g., photonic and piezoelectric devices) together, a new sector of microelectronics industry, known as micro-electro-mechanical systems (MEMS), has started to emerge. MEMS are finding ever broadening applications, including complex sensor and actuator arrays that go into devices such as air bag activators and other miniature smart material devices. Many of the processes used to fabricate MEMS devices still depend on expensive and complicated semiconductor equipment and facilities, nevertheless. These processes are largely limited to silicon-based materials. Furthermore, some of the processing temperatures for MEMS fabrication are not compatible with electronic devices. Due to the ease of processing (including lower processing temperatures), lower cost and good mechanical integrity, polymers have been considered to be ideal materials for MEMS and electronics applications.
In order to develop polymer-based MEMS, one must develop methods for depositing layers of materials comprising functionally active polymers (e.g., piezoelectric, pyroelectric, photonic, mechano-chemical, thermoelectric, etc.) and passive polymers (conducting, semiconducting, insulating, dielectric, etc.) onto one another. Piezoelectric and pyroelectric materials are particularly useful elements in the MEMS devices for sensor and actuator applications. Hence, it is highly desirable to develop a direct-write method for directly depositing a piezoelectric or pyroelectric polymer onto a solid substrate.
Certain materials are capable of being polarized when subjected to mechanical or electrical stresses. For instance, poly (vinylidene fluoride) (PVDF) can be polarized by stretching a sheet of PVDF at a temperature of approximately 70xc2x0 C. to at least three times its length, and subjecting the stretched sheet to a DC field of at least 1 MV/cm. PVDF has been a preferred polymeric material for polarization, since it has been found to have a high capability of polarization response, thereby providing high piezoelectric or pyroelectric properties or highly desired optical properties. Subjecting such a stretched film to a DC field applied in a direction perpendicular to the plane of the stretched film causes an orientation of the molecular dipoles of the materials. In the case of PVDF, the fluoro groups have a negative charge and the hydrogen atoms attached to the other carbon of the vinylidene fluoride unit of the polymer have a positive charge. Vinylidene fluoride units in a PVDF film are known to exist in at least two different crystalline forms: (1) a planar zigzag polar form or trans form (beta form or Form 1) and nonpolar and nonplanar T-G-T-Gxe2x80x2 form (alpha form or Form 2) in where T denotes trans configuration and G and Gxe2x80x2 denote the two types of gauche forms. The desired increase in Form 1 has been realized by subjecting PVDF films to stretching and subsequently subjecting the stretched films to high DC fields over extended periods of time at high temperatures. Such a treatment with a DC field is referred to as poling. The polarized material is cooled after poling for purposes of retaining the polarization. The existence of a high content of Form 1 is essential to achieving the highest amount of desired polarization properties required of good piezoelectric and pyroelectric responses.
Polarized PVDF materials are commonly used in making transducers, which utilize the piezoelectric or pyroelectric or other polarization properties of such polarized materials. It is well-known that various other polymers, such as polyvinylchloride (PVC), polyvinylfluoride (PVF), vinylidene fluoride copolymers, and many other polymers have the capability of being polarized as do a large number of ceramic materials such as lead zirconate titanate (PZT).
Mechanical stretching in the film direction causes an unequal (or anisotropic) strength in the stretching or axial direction (X-X1) as compared to the transverse direction (Y-Y1). This is an undesirable attribute of the piezoelectric polymers. Instead, it is desired to provide polymers which are substantially free of such mechanically induced orientation and which have a polarization that is stable up to the crystal melting point (if the polarized material is crystalline) or material softening point (if non-crystalline). Furthermore, the need to mechanically stretch a polymer film is not compatible with traditional microelectronics fabrication processes. In order to fabricate a MEMS device that contains a piezoelectric polymer element, it is essential to develop a direct-write method for directly depositing polarized or polarizable polymers to a solid substrate without post-deposition stretching. Scheinbeim, et al. (U.S. Pat. No. 4,830,795, May 16, 1989 and No. 4,863,648, Sep. 5, 1989) disclosed a solvent polarization method to produce a thick film of a simple geometry; however, no method has been proposed that allows direct deposition of a thin film of a polarizable material with a predetermined pattern on a solid substrate. It is desired to integrate such a direct-write method with other microelectronic fabrication methods or solid freeform fabrication methods to fabricate a multi-layer device on a point-by-point and layer-by-layer basis.
A direct-write method has been developed by which highly polarized materials in the form of a patterned thin film can be deposited onto a solid substrate. The materials deposited are substantially free of mechanically induced orientation and their polarization is essentially stable up to about the crystal melting point of the polarized material in the case of a crystalline material or up to about the softening point (glass transition temperature) of the polarized material in the case of non-crystalline polarized material. The method includes dissolving a material to be polarized in a solvent or solvents for that material to form a solution. The solution is then dispensed in the form of fine discrete droplets or a continuous strand in a fluent state and deposited onto a target surface in an essentially point-by-point and layer-by-layer fashion. The target may be a semiconducting substrate or a conducting electrode supported on the surface of a substrate. The solvent is selected which is adapted to the polarization of the material and which can be removed to the extent desired during the course of the polarization. A high electric field is applied to polarize the deposited material while the solvent is being removed. The temperature employed will be one at which polarization effectively occurs, normally at an elevated temperature below which substantial dielectric breakdown could occur. The DC field employed in the polarization will be selected which provides the desired polarization.
Also provided by this invention is a freeform fabrication method by which a multi-layer device containing a polarized material can be produced point by point and layer by layer. Individual layers may comprise a patterned thin film of a polarized material and/or other electronically active or passive materials such as a conductor, resistor, semi-conductor, capacitor, inductor, superconductor, diode, transistor, light-emitting element, light-sensing element, solar cell element, sensor, actuator, semiconductor logic element, electro-optic logic element, spin material, magnetic material, thermo-electric element, electromagnetic wave emission, transmission or reception element, electronically addressable ink, or a combination thereof. The polarizable material presently preferred is poly (vinylidene fluoride) or certain copolymers of vinylidene fluoride. A functionally responsive device such as a micro-electro-mechanical system (MEMS) can be fabricated point by point and layer by layer under the control of a computer in accordance with a computer-aided design (CAD) of this device.